Method and circuit configuration to augment the driving stability of a road vehicle

ABSTRACT

A method and a circuit configuration to augment the driving stability of a road vehicle using traction slip control by brake management. The drive wheel is identified which instantaneously exhibits the higher coefficient of friction (HM-wheel). The rotational pattern of this wheel is monitored and stabilized by brake management as soon as the traction slip of this wheel exceeds a limit value. Braking pressure which may exist in the wheel brake of the second drive wheel (LM-wheel) is decreased.

BACKGROUND OF THE INVENTION

The present invention relates to a method and a circuit configuration toaugment or preserve the driving stability of a road vehicle bycontrolling the traction slip by means of a brake management system.

To improve the starting behavior and the traction of a road vehicle, itis known to install devices on the vehicle which prevent spinning of thedrive wheels or excessive traction slip. Such spinning tendencies occuras soon as the traction torque of the vehicle engine exceeds the torquewhich is transmittable through the wheels onto the road.

With rising traction slip, it is not only the torque in the drivingdirection that is transmittable onto the road which decreases--afterhaving exceeded a maximum--but also the lateral guide force of thewheels decreases to a relatively very small amount. Due to this, thesteerability and the driving behavior during cornering is impaired invehicles with front-wheel drive. In vehicles with rear-wheel drive,there is even the risk of skidding when the lateral guide force is toosmall.

To control the traction slip, it is known from German published patentapplication 36 24 722 A1, which herein is referred to as an example of agreat number of publications relating to systems of this type, tostabilize the imminently spinning drive wheel by way of brakemanagement. In consideration of the temperature and the loading capacityof the brake, the period of time in which the brake can compensate forthe excess torque of the drive axle is very limited. For this reason,brake management is combined with engine management in other knowntraction slip control systems, for instance as described in Germanpublished patent application 38 09 101 A1. However, this entailsconsiderably greater expenditures.

DE 38 01 267 A1 teaches a vehicle brake system wherein, for augmentingthe driving stability during braking on a surface with a differentcoefficient of friction right/left, the braking pressure is reduced onthat rear wheel running on the non-skid road surface once the brake slipon this wheel exceeds a predefined value. This way, the lateral guidingforce which can be generated by this wheel is increased, and swerving ofthe vehicle is prevented. This brake system is used instead of ananti-lock brake system.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method ofcontrolling traction slip which is exclusively based on brakemanagement, the operation of which is not limited for a short period oftime in consideration of the brake load. This method should contributeto enhancing the driving stability.

It has been shown that this object can be achieved by a method by whichthe rotational pattern of a wheel (HM-wheel) is monitored and stabilizedby brake management as soon as the traction slip of this wheel exceeds alimit value and after commencement of brake management on the wheel(HM-wheel) having the higher coefficient of friction, the introductionof braking pressure into the wheel brake of the second wheel (LM-wheel)of the axle is prevented, and braking pressure existing in the wheelbrake of this second wheel (LM-wheel) is kept constant or is decreased.

Previously, traction slip control served above all to stabilize by brakemanagement that wheel of the drive axle which was relieved from load andexhibited the lower coefficient of friction. The present invention isbased on the findings that it is sufficient and useful to ensure, inevery situation and every driving condition, that the wheel driven onthe higher coefficient of friction remains stable. Traction sliptendencies on this HM-wheel are compensated for by brake management, sothat the lateral guide force of this wheel is preserved. The behavior ofthe wheel relieved from load (LM-wheel) is of secondary importance inthis situation. By using a method in accordance with the invention,further pressure increase on this LM-wheel is prevented and, as the casemay be, prevailing braking pressure is reduced.

What is important is to properly identify, at any time, the wheelexhibiting the greater lateral guide force. In a favorable embodiment ofthe invention, this wheel is identified by evaluation of test values andcriteria indicative of the rotational pattern of the driven wheels (e.g.by the chronological order of the two wheels of a driven axle becomingunstable), by comparison of the (filtered) traction slip of these twowheels, or in any similar fashion. In prior art systems, traction sliphas been controlled in a conventional manner by controlled brakeapplication of the wheel under lesser load (this type of control isreferred to herein as "traction slip control" to make a distinctionvis-a-vis the invention). In the present invention, the wheel exhibitingthe greater lateral guide force can be identified by comparison of themean braking pressure level in the wheel brakes of the two driven wheelsduring this traction slip control.

An advantageous circuit configuration for implementing the invention iscomposed of circuitry for the identification of the wheel whichinstantaneously exhibits the higher coefficient of friction, formonitoring this wheel, and for controlling the braking pressure whenthis wheel becomes unstable. This circuitry is intended as asupplementary unit to a circuit for traction slip control by brakemanagement which becomes operative when a driven wheel becomes unstableand the circuitry begins to function as soon as the vehicle speedexceeds a predetermined speed threshold or as soon as predeterminedcriteria are fulfilled which are indicative of an excessive load on thewheel brakes or on other vehicle components. Further, means are providedwhich, after brake management commences on the wheel (HM-wheel)exhibiting the higher coefficient of friction, prevent the introductionof braking pressure into the wheel brake of the second wheel (LM-wheel)of this axle, these means keeping constant, or reducing in this case,braking pressure prevailing in the wheel brake of this second wheel(LM-wheel). The above-mentioned speed threshold is suitably set to avalue between 20 and 50 km/h (e.g. to 25 km/h). At the onset of thecontrol responsive to the rotational pattern of the HM-wheel (referredto herein as "stability control" or "SK"), the traction slip controlexpediently is disconnected in this combination of traction slip controland stability control.

According to another aspect of this invention, the HM-wheel-responsivecontrol, instead of the traction slip control, starts to function assoon as the brake temperature, or an approximation of the temperatureobtained by measuring the operations determining the brake temperature(temperature pattern), exceeds a limit value.

Further features, advantages and possibilities of application of thisinvention will become apparent from the following description of anembodiment with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 are graphs showing the rotational pattern (v_(Rad)) of thetwo wheels of a driven axle, the vehicle reference speed (v_(Ref)) andthe braking pressure variation (P) in different situations which areimportant for the application of the invention, and

FIG. 4 is a flow chart of an embodiment of an exemplary method inaccordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

The diagrams in FIGS. 1 to 3 serve to illustrate the control operationsin application of the present invention.

The driving stability of a driven vehicle can be safeguarded in anydriving condition by brake management alone, provided the control isdevised such that at least the HM-wheel, (i.e., the wheel running on thehigher coefficient of friction) or the traction slip of this wheel, ismaintained within a range in which this wheel can exert a great lateralguide force. That is to say, the traction slip of this HM-wheel is keptat a low value by brake management. The second wheel of the driven axlemay exhibit great traction slip in this situation without drivingstability being jeopardized. Driving stability is preserved even if thewheel running on the lower coefficient of friction exhibits greattraction slip or races and, consequently, can receive only a minimum oflateral guide force.

FIG. 1 shows the rotational pattern of the two driven front wheels andthe variation of the applied braking pressure in a like situation.V_(RadL) is the speed of the left wheel and V_(RadR) is the speed of theright wheel. P_(R) and P_(L) are the brake pressures applied to theright and left wheels, respectively.

The vehicle speed or the vehicle reference speed V_(Ref) risescontinuously. The left front wheel is the LM-wheel in this case becauseit exhibits a comparatively low coefficient of friction (Low μ). It isdiscernible from the illustrated speed variation V_(RadL) that thiswheel tends to spin and exhibits a great traction slip. Hence, the leftwheel is able to exert only a small amount of lateral guide force. Theright front wheel, as shown by curve V_(RadR), represents the HM-wheelin this case. Traction slip tendencies of the HM-wheel are compensatedfor by the introduction of braking pressure P_(R) into the wheel brakeof this wheel. Thus, this wheel remains stable, always displays onlylittle traction slip and, as a result, is able to exert the great amountof lateral guide force desired. The wheel brake of the left, spinningfront wheel remains pressureless (P_(L) is practically zero). Brakingpressure which possibly prevailed in the left wheel at the commencementof the illustrated process, and which might be due to a precedingcustomary traction slip control operation, would be decreased at theonset of stability control in accordance with the invention.

It is important for implementing a method embodying the invention toreliably identify and select the HM-wheel. To this end, a number ofcriteria are suitable depending on the situation. FIG. 2 shows asituation in which a proper identification of the HM-wheel is possibleby determining the traction slip. At the point of time t₁, the tractionslip on the left front wheel exceeds a slip threshold λ_(O), while thespeed V_(RadR) on the right front wheel differs but slightly from thevehicle speed (or vehicle reference speed) V_(Ref) over the entirestretch of time shown. Hence, the right front wheel is the HM-wheelwhich, if necessary, is stabilized by braking pressure introduction inorder to preserve the lateral guide force. This application of brakepressure is not needed in the example shown in FIG. 2.

FIG. 3 shows a situation in which the traction and the starting behaviorof a vehicle are improved by a conventional traction slip controloperation. In performing this traction slip control operation, therunning behavior of the two drive wheels is stabilized by brakemanagement. The brake is activated as soon as an excessive amount oftraction slip occurs. Of course, an excessive amount of traction slip isencountered at first on the wheel relieved from load, the one exhibitingthe lower coefficient of friction (LM-wheel). Subsequently, the secondwheel can also become unstable so that it too must be slowed down. Brakemanagement in such a traction slip control operation can be done onlyfor a short stretch of time though, because otherwise the brakeoverheats. The brake system must be protected by engine management,(i.e., reduction of the drive torque), or by deactivation of thetraction slip control based on brake management, when the load of thebrake limit is reached.

In the present case, in lieu of deactivating the traction slip control,one switches over to a method according to the invention, referred to asstability control (SK-mode). To identify the HM-wheel as a preconditionfor switching-over to this SK-control, the mean pressure levels of thetwo wheels of the driven axle are compared when traction slip control isperformed. The wheel having the lowest mean pressure level is recognizedas the HM-wheel. At the point of time t₂, during the control operationaccording to FIG. 3, the left front wheel has the higher pressure, as isshown by the pressure variation curves P_(L) and P_(R), while thepressure level P_(R) exceeds the pressure level P_(L) at the point oftime t₃.

While a stability control operation according to the invention is beingperformed, it is not possible to identify the HM-wheel by virtue of thepressure levels. Identification of the HM-wheel is carried out in thissituation by comparing the (filtered) slip of the two drive wheels. Thewheel having the least traction slip is the HM-wheel in this case.

The HM-wheel is principally desired to be kept stable. This is equallytrue in the field of pure traction slip control, in which pressure isintroduced at once when instability of a drive wheel occurs, and alsoafter change-over to stability control according to the invention. Ifrequired, the HM-wheel is stabilized by brake management. During thestability control operation it is not necessary to build up brakingpressure on the HM-wheel permanently, namely if:

a) after slip identification on the LM-wheel, no slip has beenidentified on the HM-wheel so far, and

b) the HM-wheel maintains its stability on its own after brakemanagement.

Although the HM-wheel continues to be monitored in the previouslymentioned cases, no pressure modulation is necessary for maintaining thestable running of this wheel.

The same control criteria apply both for the HM-wheel and for theLM-wheel during the pure traction slip control.

FIG. 4 is a flow chart showing the behavior of a circuit configurationaccording to the present invention during a specific situation. Thefollowing sequence is discernible:

At step 1, the program sequence (SK-mode) described herein begins. Ifthe presence of traction slip is affirmed at step 2, and if this appliesto the HM-wheel at step 3, the introduction of braking pressure forstabilizing the HM-wheel is initiated at step 4.

At step 5, it is determined whether the braking pressure P in the wheelbrake of the HM-wheel lies below a predetermined threshold valueP_(sch). If not, the pressure is kept constant at step 6. In any case, acomparison of the slip of both driven wheels is performed at step 7.During the initiation of the stability control (SK-mode), the tractionslip of the HM-wheel (for the physical reasons explained above) must besmaller than the slip of the second driven wheel which, as is known,runs on a lower coefficient of friction. At step 7, if the instantaneousslip of the HM-wheel is greater than the traction slip of the secondwheel, there is a wrong interpretation. At step 8, the control mustreorient itself. The other wheel is the HM-wheel in reality.Consequently, following step 8, at step 9, the slip monitoring isapplied to the drive wheel which is now identified as the HM-wheel.Simultaneously, the braking pressure reduction on the second drivewheel, (which has now been identified as LM-wheel), is initiated at step10.

This program sequence repeats in every loop.

That is to say, the invention provides a simple method to augment thedriving stability of a vehicle during a traction operation and to ensurethat driving stability is preserved. This is achieved using the brakes,without engine management. As a supplement to a traction slip controlsystem which is based on brake management, the range of application ofsuch a control is considerably extended. Hence, it follows that thenecessary disconnection of the customary systems, in order to avoidbrake overload, and the ensuing disadvantages are overcome by theinvention.

We claim:
 1. A method to augment or preserve driving stability of a roadvehicle by controlling traction slip by means of a brake managementsystem, characterized in that a first wheel of a driven axle is foundwhich instantaneously exhibits a higher coefficient of friction, in thata rotational pattern of the first wheel is monitored and stabilized bybrake management as soon as the traction slip of the first wheelsurpasses a limit value, as well as in that after the commencement ofbrake management on the first wheel exhibiting the higher coefficient offriction the introduction of braking pressure into a wheel brake of asecond wheel of this axle is prevented, and braking pressure prevailingin the wheel brake of the second wheel kept constant or decreased.
 2. Amethod as claimed in claim 1, characterized in that the first wheelexhibiting the higher coefficient of friction is identified byevaluation of test values and criteria indicative of the rotationalpattern of the driven wheels.
 3. A method as claimed in claim 2, whereinthe test values and criteria indicative of the rotational pattern of thedriven wheels include the chronological order of the two first wheel andthe second wheel becoming unstable.
 4. A method as claimed in claim 2,wherein the test values and criteria indicative of the rotationalpattern of the driven wheels include a comparison of the traction slipof the first wheel and the second wheel.
 5. A circuit configuration foraugmenting driving stability, comprising circuitries for identifying afirst wheel which instantaneously exhibits a higher coefficient offriction, and comprising circuitries for monitoring the first wheel andfor controlling the braking pressure when the first wheel becomesunstable by implementing stability control, characterized in that thecircuit configuration is provided as a supplementary unit to a circuitconfiguration for traction slip control by brake management, which setsin when a driven wheel becomes unstable and starts to function as soonas the vehicle speed exceeds a predetermined speed threshold, or as soonas predetermined criteria are fulfilled being indicative of an excessiveload on the wheel brakes or on other vehicle components, and in thatmeans are provided which, after brake management commences on the firstwheel exhibiting the higher coefficient of friction, prevent theintroduction of braking pressure into a wheel brake of a second wheel,and in that these means keep constant or reduce braking pressureprevailing in the wheel brake of the second wheel.
 6. A circuitconfiguration as claimed in claim 5, characterized in that the speedthreshold is set to a value between 20 and 50 km/h.
 7. A circuitconfiguration as claimed in claim 5, characterized in that the firstwheel exhibiting the higher coefficient of friction is found bycomparison of the mean braking pressure level in the wheel brakes of thetwo wheels of a driven axle during a traction slip control operation. 8.A circuit configuration as claimed in claim 5, characterized in that atthe onset of the stability control responsive to the rotational patternof the first wheel exhibiting the higher coefficient of friction, thetraction slip control is disconnected.
 9. A circuit configuration asclaimed in claim 5, characterized in that the stability controlresponsive to the first wheel exhibiting the higher coefficient offriction instead of the traction slip control starts to function as soonas brake temperature or an approximation obtained by measuring theoperations determining the brake temperature exceeds a limit value. 10.A method to improve driving stability of a road vehicle in whichtraction slip is regulated using brake management, said road vehiclehaving a driven axle, said driven axle having at least two wheels, eachwheel having a respective brake and a respective coefficient offriction, the method comprising the steps of:(a) monitoring a respectiverotation of each of said wheels, and calculating a respective measure oftraction slip therefrom for each respective wheel; (b) determining whenone of said measures of traction slip exceeds a predetermined value; (c)identifying a wheel of said driven axle which has a higher instantaneouscoefficient of friction than the other wheel of said driven axle, saididentifying being performed when said traction slip of said identifiedwheel exceeds said predetermined value; (d) stabilizing the rotation ofsaid identified wheel using brake management, when said measure oftraction slip exceeds said predetermined value; and (e) preventing anincrease in braking pressure on said brake of said other wheel of saiddriven axle.
 11. A method in accordance with claim 10, wherein theidentifying step (c) includes:(1) determining whether one of said twowheels of said driven axle remains stable after the other one of saidtwo wheels becomes unstable; and (2) identifying said stable wheel ashaving the higher coefficient of friction.
 12. A method in accordancewith claim 10, wherein the identifying step (c) includes:(1) comparingrespective measures of traction slip from each of said two wheels todetermine which one of said wheels has a lower measure of traction slipthan the other of said wheels; and (2) identifying said wheel having alower measure of traction slip as having the higher coefficient offriction.
 13. Apparatus for improving driving stability of a roadvehicle having a traction slip control system, said traction slipcontrol system using brake management, said road vehicle having a speedand a driven axle, said driven axle having at least two wheels, eachwheel having a respective coefficient of friction, the apparatuscomprising:means for monitoring a respective rotation of each of saidwheels and for calculating a respective measure of traction sliptherefrom for each respective wheel; means coupled to said monitoringmeans for determining when one of said measures of traction slip exceedsa predetermined value; means responsive to said determining means foridentifying a wheel of said driven axle which has a higher instantaneouscoefficient of friction than the other wheel of said driven axle; meansresponsive to said identifying means for stabilizing the rotation ofsaid identified wheel using brake management; and means responsive tosaid identifying means for preventing an increase in braking pressure onsaid brake of said other wheel of said driven axle.
 14. Apparatus inaccordance with claim 13, further comprising:means coupled to saidmonitoring means and said brakes for starting said monitoring means whenan excessive load on said brakes is detected; and means coupled to saidmonitoring means for starting said monitoring means when said vehiclespeed exceeds a predetermined threshold value.
 15. Apparatus inaccordance with claim 14, wherein said threshold value is between 20 and50 kilometers per hour.
 16. Apparatus in accordance with claim 13,wherein:(1) each of said brakes has a respective mean braking pressure,and said identifying means includes means for comparing mean brakingpressures of said brakes during a traction slip control operation, and(2) said wheel having a higher coefficient of friction is identified bysaid comparing.
 17. Apparatus in accordance with claim 13, furthercomprising means for disconnecting said traction slip control systemfrom said brakes while said stabilizing means is operating. 18.Apparatus in accordance with claim 13, wherein:(1) each brake has arespective temperature, and (2) said road vehicle has a traction slipcontrol system, further comprising means for disconnecting said tractionslip control system from said brakes when the temperature of one of saidbrakes exceeds a predetermined value.